The present invention is directed, in general, to surface acoustic wave (SAW) circuits and, more specifically, to a wafer-scale package for a SAW circuit and method of manufacturing such wafer-scale package therefor.
Surface acoustic wave (SAW) devices for use in electronic signal processing have been advantageously adopted by the electronics industry. Such SAW devices have several advantages over more conventional technologies. They can be designed to provide complex signal processing in a single unit, and they also offer an additional benefit, which is their ability to be mass produced using semiconductor microfabrication techniques. These techniques lead to lower-cost devices, having only small operating characteristic variations from unit to unit. Since SAW devices may be implemented in rugged, light-weight and power-efficient modules, they find many important applications, especially in mobile, wireless and spaceborne communication systems. Such communication systems typically operate over a wide range of frequencies from about 10 megahertz to about two gigahertz. The specific signal processing capabilities and frequency range of SAW devices may be customized to allow SAW devices to perform several roles in electronic systems.
An important feature of the SAW device is its geometry, which incorporates two metal patterns having interdigitated conductive lines or traces. Such interdigitated metal structures are formed on a piezoelectric substrate and act as input and output signal paths when an AC signal voltage is applied to one of the metal structures. This AC voltage induces a surface acoustic wave in the underlying substrate wherein the acoustic wave propagates to the output structure. The interdigitated metal lines of the signal receiving portion detect the acoustic wave and convert it into a filtered electrical output signal. SAW devices, operating in the Rayleigh wave mode, can generally be designed to provide bandpass filters that achieve responses, which would otherwise require several hundred inductors and capacitors in conventional LC filter designs. Proper operation and containment of the acoustic waves require precise construction of both the central and outlying regions.
Turning briefly to FIG. 1, illustrated is a diagram of a conventional packaged SAW device 100. As illustrated, the packaged SAW device 100 includes interdigitated metal structures 110 and a piezoelectric substrate 120, both of which are manufactured on a wafer substrate 130. Formed over the interdigitated metal structures 110 and the piezoelectric substrate 120, and contacting the wafer substrate 130, is a hermetic enclosure 140. It is common for the hermetic enclosure 140 to have a substantially larger footprint than the footprint of the piezoelectric substrate 120. An aspect ratio of 6 to 1, representing a ratio of the footprint of the hermetic enclosure to the footprint of the piezoelectric substrate, is not uncommon in today""s electronics industry. Packaging the Prior Art SAW devices 100 as shown in FIG. 1 has become well accepted, unfortunately, the outermost footprint of the packaged SAW device 100 is larger than currently desired in the electronics industry, particularly the wireless telephone industry.
Accordingly, what is needed in the art is a method of packaging SAW devices that does not experience the space limitations experienced in the prior art methods.
To address the above-discussed deficiencies of the prior art, the present invention provides a SAW circuit package and a method of fabricating the package. In one embodiment, the package includes: (1) a substantially planar piezoelectric substrate, (2) SAW circuit conductors located over the substrate and (3) a passivation layer located over the SAW circuit conductors, the substrate and the passivation layer cooperating to form a hermetic seal to isolate the SAW circuit conductors from an environment proximate the package.
The present invention therefore introduces packages for SAW circuits that substantially reduce the footprint of prior SAW packages. The smaller footprint allows more SAW circuits to be employed in a given piece of equipment (such as a wireless telephone), or allows a piece of equipment having the same number of SAW circuits to be much smaller.
In one embodiment of the present invention, the passivation layer substantially alters an electrical characteristic of a SAW device formed by the substrate and the SAW circuit conductors. In an embodiment to be illustrated and described, the design of the SAW device takes into account the effect that the passivation layer has on the underlying SAW device.
In one embodiment of the present invention, the substrate comprises one selected from the group consisting of: (1) bismuth germanium oxide, (2) gallium arsenide, (3) lithium borate, (4) lithium niobate, (5) lithium tantalate, (6) langasite, (7) lead zirconium tantalate and (8) quartz. Those skilled in the pertinent art will understand that other currently-known and later-discovered materials may be suitable for use as a substrate, depending upon a particular application.
In one embodiment of the present invention, the passivation layer comprises a plurality of vias containing metal, the metal contacting the SAW circuit conductors to form terminals therefor. In this embodiment, the passivation layer actually becomes the mounting surface when the package is mounted to a circuit board. Of course, the vias (and resulting terminals) may extend through the substrate or the sidewalls, as may be advantageous to a particular application.
In one embodiment of the present invention, footprints of the substrate and the passivation layer are coextensive. This allows the package to occupy no more circuit board xe2x80x9creal estatexe2x80x9d than the bare substrate itself, which is distinctly advantageous. Those skilled in the art will recognize, however, that such is not necessary to remain within the broad scope of the present invention.
In one embodiment of the present invention, the passivation layer comprises one selected from the group consisting of: (1) Silicon Carbide and (2) Silicon Nitride. Those skilled in the pertinent art will understand that other currently-known and later-discovered materials may be suitable for use as a passivation layer, depending upon a particular application.
In one embodiment of the present invention, the SAW circuit conductors cooperate to form multiple SAW resonators. Thus, more than one SAW resonator may be included in a single package. This is advantageous when a unitary filter network is desired. Of course, this need not be the case.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.